ELECTROLYTIC SHAPING OF GERMANIUM AND SILICON 



343 



wafer at which the light is focused, one can produce dimples with a vari- 

 ,'ety of shapes and relative sizes. 



I It is obvious that the double-dimpled wafer of Fig. 8 is desirable for 

 {the production of p-n-p alloy transistors. For such use, one of the most 

 [important dimensions is the thickness remaining between the bottoms 

 of the two dimples. As has been mentioned in connection with the jet- 

 I etching process, a convenient way of monitoring this thickness to de- 

 Itermine the endpoint of etching is to note the transmission of light of 

 [suitable wavelength.^ There is, however, a control method that is itself 

 [automatic. It is based on the fact that at a reverse-biased p-n junction 

 [Or electrolyte-semiconductor barrier there is a space-charge region that 

 is practically free of carriers. When the specimen thickness is reduced 

 so that space-charge regions extend clear through it, current ceases to 

 flow and etching stops in the thin regions, as long as thermally or op- 

 tically generated carriers can be neglected. However, more pitting is to 

 be expected in this method than when etching is conducted in the pres- 

 ence of an excess of injected carriers. 



A p-n junction is a means of injecting holes into n-type semiconduc- 

 tors and is the basis of another method of dimpling, shown in Fig. 9. 

 The p-n junction can be made by an alloying process such as bonding 

 an acceptor-doped gold wire to germanium. The ohmic contact can be 

 made by bonding a donor-doped gold wire and permits the injection of 

 a greater excess of holes than would be possible if the current through 

 the p-n junction were exactly equal to the etching current. Dimpling 

 without the ohmic contact has been reported.^ 



14 



OHMIC CONTACT 



p-n JUNCTION 



Fig. 9 — Dimpling with carriers injected by a p-n junction. 



